1. Field of the Invention
This invention relates to an improved method for the preparation of powders composed of high temperature superconducting ceramics, especially those composed of oxide ceramics.
2. The State of the Art
The recently discovered and investigated perovskite superconducting ceramics (e.g., YBa.sub.2 Cu.sub.3 O.sub.7-x, also known as the "1-2-3" superconductor or "YBCO") are most amenable to processing into useful articles if first provided in the form of a phase-pure powder. Also, the preparation of high quality superconductor powders in large quantities via an economically viable process is a necessary requirement for the subsequent processing and commercial utilization of high temperature superconductors (HTSCs) in general.
To date, various production techniques have been investigated and reported in the literature. The most common method of preparing YBCO and similar HTSCs is by the solid state reaction of a physical mixture of the initial oxides: CuO, Y.sub.2 O.sub.3, and BaCO.sub.3. This technique involves calcining this physical mixture, grinding the calcination product, and repeating the calcination and grinding steps until a product of the desired phase purity is achieved. See, e.g., D.W. Johnson, et al., "Fabrication of Ceramic Articles from High Tc Superconducting Oxides," pub. in Advanced Ceramic Materials, Vol. 2, No. 3B, Special Issue entitled "Ceramic Superconductors," Amer. Cer. Soc. (1987). Such a process is time consuming as well as energy and labor intensive. Further, because an analysis or quality control step is needed to determine the results of each calcination, such a process is not readily scaled up to commercial production levels.
Another method of powder preparation is by the chemical precipitation of the appropriate salts from an aqueous solution; that is, the dissolution of the appropriate salts in water followed by the precipitation of the desired superconducting precursor. See, e.g., J.A. Voight, et al., "A Hydroxycarbonate Route to Superconductor Precursor Powders," Mat. Res. Soc. Symp. Proc., Vol. 99 (Materials Research Society, Pittsburgh, PA: 1987). However, this type of method is not considered to be commercially feasible for a number of reasons: the salts required for these syntheses typically have a low solubility, thereby requiring large volumes of solution (and concomitantly large amounts of waste) to produce a relatively lesser amount of powder; the simultaneous precipitation of the desired reactants in the desired stoichiometric ratio is sensitive to numerous variables, such as pH, temperature, salt concentration, and so forth; and the precursor powder produced by these methods often has an extremely high surface area, thus resulting in problems with the subsequent processing of such powders. One method for recovering and subsequent processing of superconducting powders by this method is also discussed by S.M. Johnson et al., "Preparation of Superconducting Powders by Freeze-Drying," Adv. Cer. Mat. (Amer. Cer. Soc.), Vol. 2, No. 3B, Special Issue, 1987, pp. 337-342.